Conventional Selective Compliant Articulated/Assembly Robot Arms (SCARA) in the market are driven through transmission systems, such as gear trains, belts-and-pulleys, chains and linkages, which are used to transmit power from the drive motors to move a load carried by an end-effector attached to the robotic arms. The motors are typically located at the base of the robot structure, and the robotic arms are moved by the transmission systems linking the drive motor to the arms.
Generally, robotic arms exhibit angular (θ), vertical (Z) and radial (R) motions in a cylindrical coordinate system. Angular motion refers to rotation of the robotic arm about a vertical axis at the position which the arm is pivoted to the robot body. Vertical or Z motion comprises vertical elevation of the robotic arm with reference to a base of the robotic system. Radial or R motion denotes extension or retraction motion of the robotic arm resulting in a straight-line motion of the end-effector which is typically attached to a distal end of the robotic arm. The rotational motion of a radial motor may be converted into linear motion of the end-effector to attain such radial motion.
U.S. Pat. No. 5,064,340 entitled “Precision Arm Mechanism” discloses an arrangement of a pulley and transmission belt system to achieve radial motion of an end-effector in a robotic arm. An arm structure of the robotic arm includes either two or three links pivotally connected to one another with an end-effector at the distal end of the distal link. Straight line movement of the pivoting mounting place of the end-effector is provided from a rotating drive wheel coaxial with the pivoting of the proximal end of the proximal link. A drive wheel arranged coaxially with the pivot of the proximal end of the proximal link causes the first and second links to pivot about their pivot axis. This pivoting causes the first link to pivot about its proximal end and causes the end-effector to pivot in the two link version and causes the second and third links to pivot about their pivot axis in the three link version.
As such, the power from a motor driver is not directly transmitted to a first pivot axis pivoted to a base of the robotic arm to a proximal link, but is transmitted to a second pivot axis before it is transmitted back to the first pivot axis to drive the proximal link. A similar transmission of power takes place from the second pivot axis to the third pivot axis which brings about straight line movement of the pivoting mounting place of the end-effector. A gearing housing may be located intermediate the ends of the proximal link to provide a gearing adjustment. The interconnected links of the robotic arm in this prior art are driven through a relatively complex timing belt transmission mechanism which makes the structure of the robotic arm complex. Therefore, accuracy and reliability may suffer.
U.S. Pat. No. 6,709,521 entitled “Transfer Apparatus And Accommodating Apparatus For Semiconductor Process, And Semiconductor Processing System” discloses a simpler arrangement of a pulley and transmission belt system comprising a first motor stacked on top of a second motor. A main transmission which is part of the main driving mechanism for rotating and extending/retracting a robotic arm connects the first motor and first to third links of the apparatus. The main transmission comprises a pair of gear pulleys disposed at the proximal and distal ends of each of the links. A timing belt extends between each pair of the gear pulleys and coaxial shafts comprised in each link.
The first link is driven directly by the first motor to rotate when the motor turns a shaft which connects the motor to the first link. The main driving mechanism transmits this rotational driving force to the third link or end-effector through an elaborate arrangement of gear pulleys and timing belts. One shortcoming of this design is that the housing of the first motor is completely enclosed and this apparatus does not ensure that electrical and service lines that drive the motors do not affect the production environment. External electrical cabling and wiring to the motors may inhibit rotation of the robotic arm about a 360° angle and may introduce foreign matter or unwanted obstructions caused by the exposed wiring. Furthermore, the first link is joined to the first motor by a relatively long and slender shaft that is coupled to a rotor of the first motor. This may cause compliance errors resulting from the bending of the shaft when the robotic arm is being driven such that the degree of rotation of the first motor's rotor at one end of the shaft may not be exactly the same as the degree of rotation of the first link at the other end of the shaft.
It is therefore desirable to devise a robotic arm structure that is able to achieve the required radial motion by implementing a simple pulley and belt transmission mechanism using only one motor to achieve radial motion, which also offers a substantially uninhibited rotational range as well as compactness.